Vane-type camshaft phasers for varying the timing of combustion valves in an internal combustion engines are well known. In a vane-type phaser, timing advance and retard chambers are formed within the phaser between inwardly-extending lobes of a generally cylindrical stator and outwardly-extending vanes of a rotor concentrically disposed within the stator. The stator is mechanically coupled and indexed to the rotational position of the engine crankshaft, and the rotor is mechanically coupled to the camshaft.
Typically, a camshaft phaser includes an oil control valve for controlling oil flow into and out of the advance and retard chambers to rotate the rotor with respect to the stator. The valve receives pressurized oil from an oil gallery in the engine block and selectively distributes oil to controllably vary the phase relationship between the engine's camshaft and crankshaft. By using pulse width modulated (PWM) control of the oil valve, cam timing is altered by command from an engine control module (ECM). In this manner, the oil control valve is a throttle and direction control valve that modulate cam position and the speed with which it changes from one position to another.
Several problems are known to exist with prior art oil-pressure actuated vane-type phasers.
First, engine oil pressure typically is relatively low at low engine speeds, and therefore at low engine speeds the response of a prior art camshaft phaser can be sluggish and not predictable.
Second, oil viscosity is temperature dependent, and therefore phaser operation at low ambient temperatures and high oil viscosity can be slow and unreliable. At high engine temperatures, as may occur in warm climates, engine viscosity can be undesirably low, resulting as above in low oil pressure.
Third, for fast phaser actuation a larger engine oil pump may be required, at a cost of additional parasitic energy drain on the engine and increased engine manufacturing cost.
What is needed in the art is a camshaft phaser system that does not rely on dynamic supply of engine oil under pressure for actuation of a camshaft rotor.
It is a principal object of the present invention to provide camshaft phasing that is independent of a dynamic supply of engine oil to the phaser.
It is a further object of the invention to provide reliable camshaft phasing over a wide range of engine speeds and operating temperatures.